Wireless communications system with detection of foreign radiation sources

ABSTRACT

The invention discloses a method for use in a wireless communications system with a plurality of broadcasting nodes, comprising the step of enabling one node in the system to function as a central node in said system and letting said node enable measurements on at least one frequency in a frequency band used by the system. Said measurements are carried out to detect if said at least one frequency is being utilized by a transmitter foreign to the system. Preferably, the measurement is enabled by means of the node transmitting a message to other nodes in the system, said message being a message pre-defined within the system as a message prohibiting all nodes from transmitting during a certain interval, said message being transmitted after the system has been detected by the node to be silent during a predefined interval between frame transmissions from the nodes in the system.

This application is the U.S. national phase of international applicationPCT/SE02/01647 filed in English on 13 Sep. 2002, which designated theU.S. PCT/SE02/01647 claims priority to U.S. Application No. 60/318,880filed 14 Sep. 2001, U.S. Application No. 60/356,404 filed 11 Feb. 2002,U.S. Application No. 60/359,326 filed 22 Feb. 2002 and 60/380,389 filed13 May 2002. The entire contents of these applications are incorporatedherein by reference.

TECHNICAL FIELD

Certain wireless communication systems, such as for example wirelesslocal area networks (WLAN) operate in frequency bands which are alsoused by radar systems. There is thus a need for such communicationssystems to be able to co-exist with radar systems, and accordingly, tocarry out measurements for the presence of radars which operate on thesame frequency band as the communications system, in the vicinity of thecommunications system.

Once the presence of a radar operating on the same frequency band isdetected in the vicinity of the communications system, a control node inthe system can control the system to take predetermined steps.

STATE OF THE ART

The coexistence of radar systems and wireless communications system onthe same frequency bands is a relatively new issue, and thus there havebeen relatively few attempts to solve this problem.

SUMMARY OF THE INVENTION

There is thus a need for a method by means of which a wirelesscommunications system can detect the presence of radar signalstransmitted on the frequency band which has been assigned to thecommunications system. The method should make it possible to initiatemeasurements at more or less arbitrary points in time, and should alsobe possible to use both in systems with a fixed infrastructure and socalled ad-hoc systems.

This need is met by the present invention in that it provides a methodfor use in a wireless communications system with a plurality of wirelessbroadcasting nodes. The method comprises the step of enabling one nodein the system to function as a central node in said system and lettingsaid node enable measurements to be carried out on at least onefrequency in a frequency band which has been assigned to the system.

Said measurements are carried out to detect if said at least onefrequency is being utilized by a device or a system foreign to saidwireless communications system.

The measurement may be conducted either by the central node itself, orby the central node requesting one or more of the other nodes to conductsaid measurements, and reporting back to the central node.

Preferably, the measurement is enabled to be carried out by means of thecentral node transmitting a message to other nodes in the system, whichmessage is a message pre-defined within the system as a messageprohibiting all nodes from transmitting during a certain interval, andsaid message is transmitted after the system has been detected by thenode to be silent during a predefined interval between transmissionsfrom the various nodes in the system.

Alternatively, the message can be conveyed within a new informationelement in the periodically transmitted beacon transmissions'.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be describe din more detail below, with reference tothe appended drawings in which

FIG. 1 shows signaling for measurements according to one aspect of theinvention, and

FIGS. 2 and 3 show signaling for measurements according to analternative aspect of the invention, and

FIGS. 4 and 5 show signaling for measurements according to anotheralternative aspect of the invention, and

FIGS. 6–8 show measurements for foreign transmission sources accordingto the invention, and

FIG. 9 shows a block diagram for detection of foreign transmissionsources according to the invention, and

FIG. 10 shows a traffic scenario, and

FIG. 11 shows timing for required measurements in one aspect of theinvention.

EMBODIMENTS

The principles of the present invention will be described below, using awireless communications system of the radio local area network (RLAN)kind according to the so called IEEE802.11 standard. In order toillustrate the problems which are addressed by the invention, theversion of this standard known as IEEE802.11a will be used, since thisstandard is specified for the 5 GHz band, a band which is also used byso called C-band radars.

Since the same frequency band is used both by the RLAN-system inquestion and certain radar systems, there is thus a need to coordinatethe use of the frequency band in question. Such coordination and rulesfor coexistence on the same frequency band is presumably agreed upon inadvance, and decided either by regulatory bodies, or by the industryitself. Regardless of which coordination functions or rules that areagreed upon, there will be a need for the RLAN system to be able todetect the presence of radars in or close to the coverage area of theRLAN system in order to apply those functions or rules, a need which ismet by the present invention. Thus, this description will first focus onhow an RLAN system which uses the present invention can detecttransmissions from foreign systems such as, for example, radars.

In the IEEE802.11 standard, transmissions are asynchronous andcoordinated using either Point Coordinating Function (PCF) orDistributed Coordination Function (DCF). Regardless of whichcoordination principle that is used, the system comprises a number ofnodes or stations which communicate with each other. DCF is a mandatorycoordination function whilst PCF is an optional coordination function.For infrastructure systems, an Access Point (AP) is a central pointthrough which all traffic to and from the stations passes. The APdetermines whether or not PCF should be used. The information isconveyed in so called Beacon messages that are sent periodically andwhich contain e.g. so called timestamp and data rate information.

For so called ad hoc networks, no AP exists and DCF is the only allowedcoordination function. In ad hoc network all stations collectivelyassist in generating Beacon messages to ensure that e.g. correct timingexists among all members of the ad hoc network. For infrastructurenetwork the present invention proposes that the AP acts as “radarcontrolling node” to detect foreign transmissions such as radartransmission. For ad hoc networks, one of the stations in the ad hocnetwork is proposed in the present invention to act as “radarcontrolling node”. In order for the “radar controlling node” to be ableto detect foreign transmissions, it needs to be able to coordinate“listening periods” throughout the system, i.e. periods during whichnone of the nodes in the system are allowed to transmit.

In one aspect of the present invention, the radar controlling node (RCN)is not only able to carry out the measurements itself, it can also usedefined messages within the system to enable other nodes in the systemto carry out radar measurements during specified intervals in time. Theresults of these measurements would then be transmitted to the RCN.

In the IEEE802.11-system, there has been proposed the use of so called“spoofing frames”, which would be a normal 802.11 frame containing theso called network allocation vector (NAV) for the transmitting node.Generally the NAV informs the other members/nodes/stations within thesystem of a point in time when the current transmission, including anacknowledgement from the receiving peer node and a possible subsequenttransmission and corresponding acknowledgement, is going to end. Astation receiving the “spoofing frame” will update its internal NAVrepresentation, thus prohibiting any transmission from the station untilthe NAV expires, which would enable radar measurements to be carriedout.

According to one aspect of the invention, the measurement period duringwhich the AP could detect the presence of radar or other foreigntransmissions could be achieved by letting the AP transmit a spoofingframe, and then to carry out measurements instead of transmitting duringthe time interval specified in the NAV in the spoofing frame. In orderto ensure that the measurement is carried out at the desired point intime, the invention proposes to give high priority within the system tosuch spoofing frames. This can be done in the way outlined below.

According to the standard, before the AP is allowed to transmit aspoofing frame, it must detect that the wireless media (WM) is idle fora certain predetermined time in order to ensure that no other node wantsto transmit. This “quiet period” is known as DIFS (DistributedInterframe Space), and can also comprise a so called back-off time,(BO).

According to the invention, the AP transmits the spoofing framecontaining the measurement data after it has detected “quiet” within thesystem during a period shorter than the DIFS, or shorter than DIFS+BO.Thus would give the AP the highest priority within the system formeasurement periods, and accordingly would ensure that the measurementscan be carried out at the desired points in time.

If an even higher priority is desired for the measurement periods, the“quiet” period necessary for the AP before it transmits the spoofingframe containing the measurement data can be shortened to the so calledPIFS (PCF Interframe Space), or the SIFS (Short Interframe Space).

As an alternative to transmitting “spoofing frames” containing thedesired measurement data period, the “radar controlling node”, eitherthe AP in a infrastructure BSS (Basic Service Set) system, or a nodewhich has been appointed to act as “radar controlling node” in an ad-hocsystem (or Independent BSS), can use the so called “beacon message” inthe system to inform the other nodes/stations/subscribers in the systemof when the measurement will start, and the duration of the measurement,and also any possible information regarding repetition intervals of themeasurement periods, all of which is illustrated in FIG. 1.

The Beacon message contains fixed fields such as e.g. the Timestampfield and so called ‘Information Elements’ (IE:s), each of which definesa predefined set of information, e.g. Frequency hopping parameter setinformation. Depending on which information which is valid the beaconwill contain different IE:s. When a measurement period is required theBeacon would comprise an IE which relates to a silent period used forradar measurements, with the parameters described above.

The beacon message is transmitted periodically, and thus the informationregarding the start of the measurement period can be “counted down” orupdated with each beacon transmission. An advantage of using the beaconmessage for the desired purpose is that the beacon message istransmitted repeatedly, which minimizes the risk that other nodes in thesystem might not receive the message properly due to, for example, radioshadow.

As an alternative to either of the two embodiments described above, theRCN can, according to the invention, carry out measurements during quietperiods which occur “on their own” in the system, i.e. without anycontrolling by the RCN. According to the standard, data is sent withcertain legitimate frame sequences, with a certain minimum time intervalbetween frames e.g. DIFS, and certain time interval between the protocoldata units within a frame, e.g. SIFS. There is thus always a certainminimum quiet period between transmissions during which it would bepossible to carry out measurements in order to detect foreigntransmissions. In addition, in most systems, quiet periods which arelonger than those specified should occur naturally. In this alternative,the AP could initiate measurements as soon as it detects that the systemis quiet, and continue until a subscriber in the system starts totransmit.

Until now, this description has dealt with how, according to theinvention, it is possible to achieve periods of time during whichmeasurements for radar signals can be carried out by the AP in the IEEE802.11 system. Another issue, which is also addressed by the presentinvention, is how the detection of radar signals as such is carried outduring the “quiet periods” which have been achieved by means of theinvention.

Detection of foreign signals may be based on Received Signal Strength,RSS. If the received signal reaches a certain RSS-level, it is takennotice of by the AP, and analyzed to see if it emanates from within theRLAN-system or not.

One possibility of analyzing a signal which is received during a quietperiod ordered by the AP is to analyze the signal to see if it exhibitscomponents which show that the signal belongs to a transmission sourcein the RLAN-system in question, in this case the IEEE 802.11 system. Ifsuch components are detected, it can be assumed that the transmissionsource is not a radar system. If, however, no RLAN-components aredetected, it is assumed that a radar transmitter has been detected, andthe appropriate specified steps are taken.

The analysis described above may include attempts to detect framepreambles inherent to frames in an 802.11-system, or even attempts todecode the signal as if it were an 802.11-signal, to see if valid datais detected.

The man skilled in the field will realize that there are many other waysof checking whether or not a signal emanates from a certain system ornot, and thus, this description will not go into the details of everysuch detection possibility, they are all naturally within the scope ofthe invention.

However, one more possibility of detection of RLAN-components in areceived signal will be mentioned here: the IEEE 802.11-frames contain aso called duration field, the NAV field, i.e. a field specifying theintended length of the forthcoming acknowledgement transmission from thepeer station plus a subsequent transmission and its correspondingacknowledgement. Any signals detected during this period which are abovethe RSS-threshold can be discarded, so that only signals extendingoutside the duration of the frame are analyzed. Apart from the NAVfield, together with the preamble, a length field is included thatspecifies the length of the current protocol data unit. This lengthfield could also be used.

Returning now to the issue of measurement periods, it is naturally adesire to let the AP schedule these measurement periods at points intime when the data transmission need within the system is low, as themeasurement periods will block data transmissions.

One way of scheduling the measurement periods is to let the centralnode, the AP, monitor its own transmit buffer status, and to alsoestimate the transmit buffer status of the other nodes within theRLAN-system. One way of estimating the transmit buffer status of theother nodes within the system is to let the AP sense the media for atime longer than the longest time specified between transmissions beforesubmitting a measurement frame according to any of the methods outlinedabove. In the IEEE 802.11-system, this longest time would translate intothe sum of the maximum interframe space (IFS) and the longest backoff(BO) time.

Naturally, if the system permits this, the AP should poll all othernodes within the system for their transmit buffer status. One way ofdoing this is to utilize the beacon message in the system to show thatthe RCN uses PCF, or if a polling function is supported by thestations/nodes/subscribers, they can be polled for pending datatransmissions prior to the RCN signaling for measurement periods.

Another method could be based on counting the traffic to and from the APduring one Beacon period. If the traffic intensity is high the amount ofmeasurement time for radar detection will be set low for the next Beaconperiod, and vice versa if the traffic intensity is low.

No matter which method is used to schedule the measurement periods, theyshould be kept as short as possible, in order to minimize transmitdelays for data. A suitable value for a measurement period, given as anexample only, is two milliseconds (2 ms).

It should again be stressed that the invention is equally applicable tosystems which have a node appointed as AP from the beginning, as well asto systems which are so called ad-hoc systems. In such systems, alsoknown as IBSS-systems, there is no AP that can act as centralcontroller.

One proposal according to the inventions is to let one of the stationsin the IBSS act as the “Radar Controlling Node” central controller forthe purposes of scheduling and carrying out the measurements accordingto the inventions, as outlined above. This should be done according to apredetermined algorithm or protocol, and will transform the IBSS, forthe purposes of the radar measurements, into an infrastructure basedBSS, and thus, the same methods as for a system with a predetermined APcan be used.

One possible way of appointing one of the nodes to act as AP for radarmeasurement purposes is to assign this role to the station thatinitiates the IBSS.

In one aspect of the invention, it would, for example, be envisioned tolet the RCN be the only node that has the capacity for “activescanning”, i.e. the AP (or the node that has been assigned the role ofthe RCN) is the only unit that may initiate the use of the medium.

In another aspect of the invention, it would also, for example, berequired for a station to determine an existing “radar controlling node”either in an ad hoc network or in an infrastructure network prior toattempting any transmissions. By detecting a RCN first, it can beassumed that the frequency used by the ad hoc network or infrastructurenetwork is free from being used by a radar.

In order to detect radar-like interference signals with periodic signalcharacteristics, apart from what has been described above, it might alsobe desired to randomize the measurement or silencing intervals, therebymaking measurement intervals non-periodic. This will increase theprobability of detection of radar-like interference signals. This wouldbe applicable to all RLAN systems, even to those who are centrallycontrolled like HIPERLAN/2 and HiSWANa.

One problem of introducing randomized quiet periods is that somestations belonging to a BSS (or IBSS) may fail to receive correspondingquiet control information being sent out in the Beacon message, saidmessage having been described above. (Another solution is that stationsare permitted to send only if the most recent Beacon was correctlydecoded.) Hence, there is a risk that the quiet period is damaged by oneof the stations within the BSS (or IBSS) as it may transmit during thequiet period.

The solution to the apparent robustness problem is to introduceredundancy, which can be done in several ways. The most straightforwardmethod is to repeat information simply by indicating multiple quiettimes in each Beacon.

Each indication would then represent quiet times for different butconsecutive Beacon intervals. In order to limit the number of Quietoffset indications, the Quiet Offset fields would be cycled through overtime. To simplify implementation, the list of Quiet Offset indicationscould be permutated clockwise one step, or every sent beacon. The quiettime is referenced to the TBTT (target beacon transmit time) time, butmay also use other references such as the Beacon transmit time. Adheringto a known frame format structure, this is depicted in FIG. 2.

An example of the first embodiment is further depicted in FIG. 3. Here,a list of three different offsets is shown. It is further shown that asecond phase of cycling through the list takes place.

One drawback with the method depicted in FIG. 2 is that multiple quiettime indications in a Beacon causes unnecessary overhead. One remedy tothis inefficiency is to indicate a state for a pseudo-random generatorin each Beacon. (The random generator may be implemented as a shiftregister with feedback. The generator polynomial should be selected suchthat pseudo random characteristics of the generator output is granted. Amaximum length (linear) shift register may be a suitable choice.)

Each station can then synchronize their respective pseudorandomgenerator to the state from a correctly decoded Beacon. This state isthen used when deriving the offset (i.e. the start time) for the quiettime. Note that the quiet duration must also be indicated. Here, it isassumed that the duration remains the same. The equation below describesone method of determining the offset time for the quiet period.T _(Quiet) _(—) _(Offset) =T _(Quiet) _(—) _(Duration)×Rem(State,floor(T _(Beacon) _(—) _(Interval) /T _(Quiet) _(—)_(Duration)))where T_(Offset) is the offset time, State is the random generatorstate, T_(Duration) is the quiet time duration, T_(Beacon) _(—)_(Interval) is the interval between the Beacons and Rem is the remainderfunction.

This relation provides a set of non-overlapping quiet time instancesdistributed over the entire Beacon interval. It should be noted thatother functions might equally well be applied to determine the offsettime based on the random generator state as one of the inputs. Theparameters in the relation above are depicted in FIG. 4.

The frame format based on distributing a random generator state isdepicted in FIG. 5.

Randomised quiet periods can by introduced in centrally controlledsystems like HIPERLAN/2 and HiSWANa by the scheduler in the controllingnode or AP. In these systems no protocol is needed.

As described above, various methods are proposed by the invention inorder to differentiate between transmissions received during themeasurement periods (“quiet periods”), so that radar transmissions maybe detected.

A solution to this distinction has been described above capitalizing onthe fact that portions (frames) of RLANs have different (longer)duration than radar pulses. In the following D_SHORTEST_RADAR andD_LONGEST_RADAR are the shortest and longest radar pulse, respectively,which can be detected by the RLAN, and D_SHORTEST_FRAME is the shortestframe of the RLAN traffic. Typical values, given as examples only, are:D_SHORTEST_RADAR ˜50 ns, D_LONGEST_RADAR˜20 μs and D_SHORTEST_FRAME=24μs. Consecutive RLAN frames are separated by silent periods of variableduration, depending on the current traffic load.

If a frame with RSS>RSS_TH_1 and with duration D_I<D_SHORTEST_FRAME isdetected, then radar can be assumed, and with D_I>D_LONGEST_RADAR, thana received RLAN frame is assumed.

Because the shortest radar pulses which have to be detected have aduration in the range of 50 ns, the radar detection algorithm must bealso able to detect such short pulses.

Current RLANs use OFDM as modulation technique. This modulationtechnique is characterized by a non-constant envelope of the transmitsignal with a high dynamic range (˜10 . . . 12 dB), which causes a highvariations of the received field strength RSS.

It is assumed that the mean received signal strength of RLAN traffic canbe in the range of the radar detection power threshold RSS_TH_1. Thisthreshold determines the power level above which the RLAN has to detectradar pulses.

This high variation of RSS makes it difficult to determine the durationD_I of an interference with RSS>RSS_TH_1 (see FIG. 6) if the RLANtraffic is received within the range of the radar detection powerthreshold (+−10 dB). Unfortunately, the high variation of RSS caused bythe OFDM modulation technique may mislead to the detection of a train ofshort pulses with duration shorter than D_SHORTEST_FRAME, which may thenbe interpreted as radar pulses, instead of a single RLAN frame. Thisfalse radar detection may cause a high rate of frequency changes, or mayeven cause a longer absent time of the whole RLAN, when with proceedingdetection time all channels, which the RLAN is allowed to use, are(falsely) marked as occupied by radar.

FIG. 6 shows an RLAN frame 10, a number of peaks 20 within this frame,which are misinterpreted as radar pulses, and also shows a true radarpulse 30.

On the other hand if the RLAN looks for periodic structures in order toidentify by this characteristic a radar signal, the fluctuations of theOFDM envelope will destroy the periodic structure of a received radarsignal when it is received in the range of RSS_TH_1 and therefore theradar detection probability will decrease significantly.

According to one aspect of the invention, the pulse detection isaccomplished on the basis of mean values <RSS>_LONG, which is taken overa certain number of RSS-samples each of duration e.g. 50 ns. Caused bythis averaging process, there remains the risk that short radar pulsesin the neighbourhood of low interference are hidden and cannot bedetected. Therefore, a second measuring process is accomplished inparallel by the RLAN, which uses shorter averaging periods <RSS>_SHORT.The results of both measuring processes are combined together, in orderto allow a secure and reliable detection of radar pulses.

FIG. 7 shows the result for the case that the RLAN frame is receivedwith high field strength RSS. During the reception of the RLAN frame,enough consecutive averaged RSS values <RSS>_LONG are all above thesecond threshold RSS_TH2, thus indicating that this signal is an RLANframe. In FIG. 7, the outlines of FIG. 6 are shown with dotted lines, inorder to highlight the difference.

A preferred embodiment is that N=11 consecutive <RSS>_LONG valuesexceeding the RSS_TH_2 threshold are required for the decision that itwas an RLAN frame. In FIG. 7, for simplicity only 8 consecutive<RSS>_LONG values are sketched. The radar pulse is correctly detectedbecause the RSS_SHORT value is above the threshold RSS_TH_1 and notenough <RSS>_LONG values are above RSS_TH_2.

FIG. 8 shows the result, for the case that the RLAN frame is receivedwith low field strength RSS. During the reception of the RLAN frame, allaveraged RSS values <RSS>_Long are below the second threshold RSS_TH2and no value <RSS>_SHORT is above the threshold RSS_TH_1. Therefore, noradar signal is detected during this period. This shows that thedifference between the thresholds RSS_TH_1 and RSS_TH_2 must at least beequal to the dynamic range. The dynamic range is defined as thedifference between the average power and the peak power of an OFDMsignal, and is shown in FIG. 8 with arrows. As before the radar pulse iscorrectly received because the RSS_SHORT value is above the thresholdRSS_TH_1.

FIG. 8 also demonstrates the dependency of the threshold RSS_TH_2 fromthe threshold RSS_TH_1. RSS_TH_2 must be by D+margin lower thanRSS_TH_1, where D means the dynamic range of the OFDM signal and themargin means the dynamic range of the mean values <RSS>_LONG. RSS_TH_2must be defined such that if some or no <RSS>_LONG values are belowRSS_TH_2 then with a sufficient high probability no RSS_SHORT value isabove RSS_TH_1 if there is only an RLAN signal present.

FIG. 9 shows the block diagram for the radar detection control, where inthis example it is again assumed that D_LONGEST_RADAR<11 consecutiveRSS_LONG values<D_SHORTEST_FRAME.

The radar detection device has to control that enough time, where noRLAN traffic occurs (silent period), is available in order to be able todetect radar with high probability within a certain time. If too littlesilent time was recognized by the radar detection device, e.g. becauseof a high traffic load, then the radar detection device preferably hasto include a so called forced silent period without any RLAN traffic.This can e.g. be accomplished by delaying own RLAN traffic. (time drivenforced silent period).

Such a forced time period without RLAN traffic can not only be triggeredby a certain time constraint, as mentioned in the paragraph above, butit can also be triggered by a certain event (event driven forced silentperiod). Such an event may be preferably an event, where the radardetection device recognizes a certain uncertainty about the radardetection decision. Then it can postpone the decision, inserts a forcedsilent period, accomplishes further radar measurements within thisforced silent period, and decides during or after the forced silentperiod if radar is present or not. Such an event can e.g. be a too highnumber of corrupted RLAN frames within a certain time period T1. Anotherevent can e.g. be a too high number of detected radar pulses within acertain short time T2, which seems to be untypical for radar signals.

Below, a description will be given of this aspect of the invention, onthe basis of an example where <RSS>_LONG is averaged over 2 μs and<RSS>_LONG is averaged over 0.1 μs. Furthermore, it is assumed thatD_SHORTEST_FRAME=24 μs and D_LONGEST_RADAR=20 μs, but of course e.g. anaveraging time for <RSS>_LONG=4 μs would be a reasonable value as well.

If at least N_TH=11 consecutive values of <RSS>_LONG are all above asecond threshold RSS_TH_2, then the interference is interpreted as RLANframe. If less then N_TH=11 consecutive values of <RSS>_LONG are allabove this threshold RSS_TH_2 and at least one value of <RSS>_SHORT isabove the threshold RSS_TH_1, then the high interference is interpretedas radar pulse.

RSS_TH_2 is preferably lower than RSS_TH_(—1). RSS_TH_2 depends on thesensitivity level of the RLAN, on the dynamic range of the OFDM signaland on the averaging period for <RSS>_LONG. RSS_TH_2 can be the lowestlevel, where an RLAN signal can be successfully detected. Preferably itshould be more than the dynamic range of the OFDM signal below RSS_TH_1.E.g. if RSS_TH_1=−61 dBm and the dynamic range of the OFDM signal=12 dBthen RSS_TH_2 should be below −(73+margin) dBm. The margin should takecare of hardware inaccuracies and variations of the mean signal powerduring one RLAN frame. E.g. margin ˜5 dB. This should ensure that no<RSS>_SHORT value exceeds the RSS_TH_1 if less than N_TH=11 <RSS>_LONGexceeds RSS_TH_2 just due to RLAN traffic.

RSS_TH_1 depends on the averaging period of <RSS>_SHORT. E.g. if thereceived field signal strength radar detection threshold is −61 dBm andthe detection of a 50 ns pulse is required and the <RSS>_SHORT averaginglength is 100 ns than the RSS_TH_1 must be 3 dB below −6 dBm.Additionally, hardware inaccuracies shall be taken into accountrequiring to lower the RSS_TH_1 furthermore.

The averaging of <RSS>_LONG and <RSS>_SHORT can be accomplished overnon-overlapping time periods or over a sliding window. The above examplerelates to non-overlapping time periods. If <RSS>_LONG is determinedover a sliding window then the threshold N_TH must be increased, suchthat N_TH consecutive values <RSS>_LONG covers a time period T of thelengthD_LONGEST_RADAR<=T<=D_SHORTEST_FRAME.

The aspect of the invention presented above is simple to implement, andallows a reliable distinction of radar pulses from RLAN frames. Thedetection of RLAN frames is independent of if the RLAN frame iscorrupted (e.g. caused by collisions) or not. This is of significantimportance, because RLAN frames can also be distinguished from radarpulses by decoding the RLAN preamble. If a preamble is detected, thenthe interference is identified as RLAN frame. This method of using thepreamble for the decision does not work if the RLAN frame is corrupted,e.g. by a collision with another RLAN frame, which can frequently occur.

As mentioned a number of times previously, one of the problems addressedby the invention is to discover radar signals during periods where theRadar Detecting Device (RDD) does not transmit,. This process can besplit into two stages; measurements and detection. Simple measurementsof RSS may be satisfactorily, which puts the focus on finding efficientdetection algorithms using those measurements.

The requirement should be fulfilled that, generally spoken, P percentwithin each time interval of duration T is used by an RLAN radardetection device to scan for radar signals. During these radarmeasurement periods, interference from other RLANs should not cause afalse alarm, i.e. the RLAN radar detection device should not assume ithas detected radar although there was no radar signal present just dueto the RLAN co-channel interference. At the same time the own RLANtraffic shall be impacted as little as possible. It must be remarkedthat own traffic is not always predictable because the RLAN has also toreact on requirements coming from the distributing system (DS) or otherStations (STA). An exemplary traffic scenario is sketched in FIG. 10.

The percentage P of T can also be expressed by an absolute timeT_S=T*P/100 to be measured within T.

The basic idea of this aspect of the invention also applies if therequirement for radar detection is not a specific required measurementtime within a certain time interval but a performance requirement, e.g.the RLAN network or cell has to leave the frequency once a radar ispresent within a certain time.

The aspect of the invention which will now be described illustrates howthe radar detection device (RDD) can efficiently detect radar duringnormal mode of operation. It is assumed that the requirement for radardetection during normal mode will be that for a certain amount of timee.g. 5% . . . 20%, radar has to be measured within a certain timeinterval T. Another possibility for a radar detection requirement couldbe that the RDD has to initiate and control a frequency change or atleast take care that the RLAN network or cell does not continue itsoperation on the used frequency within a certain time after a radarsignal has become present on a certain frequency (channel). This solvesthe problem of how the RDD can efficiently control this requiredmeasuring time period during normal mode of operation withoutintroducing unnecessary traffic restrictions.

The basic concept is that the device responsible for radar detectiontracks the whole RLAN traffic firstly without actively controlling thetraffic. At the same time the radar detection device scans for radar intime periods without traffic. This scanning is performed by RSSmeasurements, which are compared with the radar detection powerthreshold RSS_TH. If this threshold is not exceeded it can be assumedthat no radar is present.

If the threshold is exceeded, the RDD checks whether this was due toRLAN traffic or not. If the RDD finds out that the exceeding was due toRLAN traffic the corresponding RLAN traffic duration is excluded fromthe measurement time because it cannot be guaranteed that not a radarsignal above the threshold was present at the same time.

If the own RLAN traffic load is too high, or if the received RLANtraffic with RSS>RSS_TH occurs too often, so that there is a risk thatthe required amount of time cannot be measured within the time period Tor the performance requirement cannot be fulfilled, the RDD starts tocontrol the traffic. This control is possible by several means. By thiscontrol it can be ensured that the radar measurements are no longerdisturbed by RLAN traffic and therefore enough time for mostlyundisturbed radar measurements is available.

If signals above RSS_TH are received, which cannot be detected as RLANtraffic the RDD has the indication that a radar signal is present andstarts to initiate and control that this frequency is no longer used.

The RDD can also start to control the traffic to suppress RLAN trafficif it notices many RSS measurements above RSS_TH although no RLANtraffic is detected. By this the RDD can ensure that it does not falselydetect radar due to non-detectable RLAN interference, caused by e.g. toomany collisions. This is of course only possible if the requirement forthe radar measurement time or performance requirement is stillfulfilled.

In the following nine steps, the idea is exemplarily described indetail. An interfering RLAN device is here denoted RLAN_I, where theindex ‘I’ denotes ‘interfering’. It is further assumed that ‘highinterference’ always means an interference with RSS>RSS_TH.

1. At the beginning of the time interval T the RDD uses each timeinterval, where the RDD does not transmit nor receive from any otherRLAN device, for radar measurements.

2. The RDD counts the time T_M really used for measurements during thetime interval T, i.e. where no high interference occurred.

3. High interference caused by another RLAN_I is excluded from T_M.

4. The RDD internally defines a time T_I<T, which depends on the currentmeasurement time T_M. I.e. T_I is continuously updated. T_I is thecloser to T the closer T_M to T_S is.

5. As long as T_M is smaller than T_S and the elapsed time reaches T_I,the RDD reserves the remaining time ΔT=T−T_I for the so-called forcedmeasurement period. In this period of the forced measurements own RLANtraffic is not transmitted and the available time ΔT is used foraccomplishing the rest of the required radar measurements. Within ΔTco-channel interference from other RLANs shall be suppressed (see item7). I.e. the medium is silenced by e.g. the RDD or another device, whichcommunicates with the RDD. This could be a device with the only task tosilence the medium if told by the RDD.

6. T_I is adapted to the capability of the RDD or the device, whichsilences the medium so that it can accomplish the remaining time T_S−T_Mof the required radar measurement time within the time interval ΔT usedfor forced measurements.

7. For IEEE RLANs, the suppressing of high co-channel interference fromother RLANs during ΔT can be accomplished either by using the RTS/CTSmechanism of IEEE802. 11. Other embodiments to silence the medium mayalso be envisioned, such as letting the device which wants to silencethe medium transmit short dummy pulses with preferably no information,with a period shorter than the shortest possible frame space of devices,which should kept be quite (preferably DIFS, but PIFS is also possible).Additionally, a beacon transmitted from the RDD could indicate to allassociated STA:s that a period where no traffic is allowed will follow.This could be a contention free period. During this period all deviceswill be quite unless they are asked to transmit something. During theseperiods the RDD can silence the medium by simply not demanding anytraffic. During these periods the radar detection could be performed.For H/2 RLANs it is not a problem to suppress such high co-channelinterference from other H/2 devices belonging to the same cell byutilizing the capabilities already foreseen in the H/2 standard.

8. As soon as the real measurement time T_M is equal to or larger thanthe required measurement time T_S, the measurements are stopped for thistime interval T, because sufficient long measurements within T have beenaccomplished.

9. As soon as radar is detected, the measurements are terminated and theRLAN changes frequency or at least stops the transmission on thecurrently used frequency

The decision, if high interference is caused by radar or by anotherRLAN_I can be accomplished in different ways and can suitably becombined together.

It is assumed that D_SHORTEST_FRAME and D_LARGEST_FRAME is the min. andthe max. length, respectively, of a RLAN frame, and thatD_SHORTEST_RADAR and D_LONGEST_RADAR is the min. and the max. length,respectively, of a radar pulse to be considered.

Typical values are e.g. D_SHORTEST_FRAME≈24 μs, D_LONGEST_FRAME≈3 ms.D_SHORTEST_RADAR≈0.05 μs and D_LONGEST_RADAR≈100 μs.

The duration D_I of high interference is measured. Based on D_I it canbe firstly decided if D_I can be seen as radar pulse or as RLANinterference.

In the following an example is shown of how the general idea to explorethe information of the length of the interference could be implementedand to distinguish by this between RLAN traffic and radar signals.

If the duration D_I>D_LONGEST_RADAR or D_I<D_SHORTEST_RADAR, then theinterference can be identified as no radar signal and therefore e.g. asRLAN interference, because such long or short radar pulses are eithernot possible or very unlikely. In this case it does not matter to knowthe originating source of the interference because is was identified asto be no radar signal.

In the other case with D_SHORTEST_RADAR≦D_I≦D_LONGEST_RADAR, then aradar pulse is possible. If further D_I is smaller thanD_SHORTEST_FRAME, then radar is detected, because such short RLAN framesare not possible or very unlikely.

If D_SHORTEST_FRAME≦D_I≦D_LONGEST_RADAR either a RLAN frame or a radarpulse is possible. Then the RDD can try to detects the preamble at thebeginning of the high interference. If a preamble can be detected, thenthe interference is identified as RLAN interference. The preamble can bea preamble from the same RLAN system or from another RLAN system (e.g.an IEEE RLAN can detect an IEEE preamble, an H/2 preamble and allpreambles from known RLAN systems and vice versa, i.e. H/2 can detectH/2, IEEE and all preambles from known RLAN systems and.

In general all RLAN systems should be able to identify all preamblesfrom al other known RLAN systems operating in the same frequency band.).If a preamble is detected, the RDD continues measuring, but excludes D_Ifrom the current measurement time T_M. I.e. no decision to leave thefrequency is dons, if this is still in line with the radar detectionrequirement. If no preamble can be detected it is assumed that theinterference signal was a radar signal.

This fact that the typical radar pulses have a duration that issignificantly shorter than the portions (frames) of data transmitted ina RLAN system could be used in another embodiment of the invention. E.g.of D_I<D_SHORTEST_FRAME than radar is assumed to be present and ifD_I>D_SHORTEST_FRAME than the interference is considered to be an RLANsignal.

For all embodiments it cannot be ensured that if an RLAN signal isdetected (by pulse length or by preamble detection) that no radar washidden in this detected RLAN signal. Therefore, the decision whetherthis time is excluded from the measurement time, which is the preferredembodiment or whether the frequency has to be vacated anyway depends onthe radar detection requirement. In any case the RDD can use theinformation that an RLAN signal above the power threshold RSS_TH wasdetected. Another possibility than excluding the time from themeasurement time is to vacate this frequency (channel) but to recheckthis frequency more frequently whether it is still occupied by highinterference signals.

Restrictions can be added in order to lower the false detectionprobability, on behalf of detection sensitivity. For instance:

-   -   The maximum interference level of the pulse must be above a        certain threshold.    -   More than one pulse must be received within a given period.    -   More than one pulse must be received within a given period, and        they shall have a common PRF (Pulse Repetition Frequency). Note        that the PRF detector can be robust regarding lost pulses.

No measurement overhead is required, and it is possible to perform radarscanning on channels that are not used. Radar scanning can be performedcontinuously (except during the duration of transmitted RLAN frames).

The described method of distinguishing radar from RLAN data by means ofthe length D_I of the received high interference and an preambledetector can be used not only during the normal mode of the RLAN butalso during the start-up phase of the RLAN.

The result of detecting a radar on a specific channel, using e.g. themethod described herein, has the result that the RLAN device will markthis specific channel as occupied by radar, and consequently move to adifferent channel, where normal operation is resumed. This could havethe consequence that a significant number of channels are being markedas occupied, leaving few or no remaining channels left for the RLANdevice to operate in. In such an event, the system capacity will suffer.

It is then clearly desirable to have a mechanism where the radar-markedchannel is measured again; This way, it becomes possible to either (i)confirm the presence of radar on the channel, or (ii) obtainmeasurements which indicate that there is no longer a radar present. Inthe latter case, it will be allowed for the RLAN device to once againoperate in the previously radar-marked channel.

In order to have a sufficiently high probability of detecting thepresence of radar on such a marked frequency, it will be required tocollect measurements over long time-intervals, orders of magnitudelonger than the measurement periods typically used for radar detectionin the normal mode, as described previously. A typical value would bethat it is required to measure for a total time of T_(TOT)=10 s. If noradar signal was seen during this entire period, the channel can—onceagain—be considered as radar-free. Such long measurement intervals arehighly undesirable, as it requires the RLAN device to leave its normaloperating mode and thereby adversely affecting the normal operation ofthe device to a high degree.

As an alternative to scheduling such long measurement intervals, one caninstead use the approach described in what follows.

The RLAN device will regularly schedule short measurement periods on itscurrent operating channel, as well as on other channels. Suchmeasurement periods are a part of the normal operation of the RLANdevice, and the purpose is to always use the channel with the bestcharacteristics (i.e. the least level of interference). The idea is thento also carry out such measurements on channels previously marked asradar-occupied, and to keep track of the total measurement time. Thetotal measurement time is defined as the sum of all shortmeasurement-intervals on the specific channel. For the followingdiscussion, each (short) measurement interval is referred to with thevariable T_(meas), while the total measurement time is referred to withthe variable T_(TOT). The variable T_(TOT) is initially set to a valueof zero once radar has been detected on a specific channel. Each time achannel previously marked as radar-occupied is measured, one of twothings can occur;

Radar is detected during the measurement interval. In this case, thechannel will keep its “radar-tag”. The value of T_(TOT) will remainzero, or no radar signal was detected during the measurement interval.In the latter case, the total measurement time is increased in acumulative manner, as T_(TOT)=T_(TOT)+T_(meas).

The effect of this scheme is that the total cumulative measurement timeon each specific channel is kept track of. In the event thatsufficiently many measurements have been carried out on the radar-markedchannel, so that T_(TOT) satisfies the given requirement on totalmeasurement time (e.g. 10 s used in this example), the previouslyradar-marked channel can once again be considered as radar-free.Furthermore, in the event that radar is detected after a number ofradar-free measurements, the cumulative total time T_(TOT) will onceagain be set to its initial value of zero. Using the method described,it will be possible to

-   -   assure the presence or absence of radar on a previously marked        channel with a sufficiently high level of probability, and that        this is achieved without the adverse affect of long continuous        measurement intervals.    -   make channels available for use by the RLAN, that would        otherwise be unavailable, thereby improving system capacity.

A possible technical solution is described in the following.

At the beginning of each time interval of length T, the counters C0, C1and C2 are reset to zero. The counter unit is comparable to the time,e.g. measured in ns. C0 is a counter for the total elapsed time duringT.

The counter C0 counts the elapsed time within the time interval T (fromzero to T).

The counter C1 counts the time already used for radar measurements(equivalent to T_M), i.e. the counter C1 is set active when anymeasurement interval starts and C1 stops counting when either the RLANstarts transmission or receiving a high interference signal withRSS>RSS_TH, or if the required amount C1==T_S of measurement time duringT is reached.

The counter C2 is optional and counts the duration of uninterrupted highinterference. If D_SHORTEST_RADAR≦C2≦D_SHORTEST_FRAME, then radar isdetected. If D_LONGEST_RADAR<C2, then RLAN interference is detected, thecounter C1 is set to active again after the end of the high interferenceand the measurement continues. If D_SHORTEST_RADAR≦C2≦D_LONGEST_RADAR,then the RLAN tries to detect the preamble within the high interference.If the preamble cannot be found then radar is detected. Otherwise thecounter C1 is set to active again after the end of the high interferenceand the measurements continue.

It is assumed that the RDD is capable to accomplish forced measurement(‘forced’ means the RDD or another device suppresses own transmissionsand transmissions of neighboring RLAN devices using the same frequencyas RDD) in X percent of the time interval of duration ΔT. Then a timethreshold T_I is set, e.g. T_I=(T−T_S−T_M)*100/X.

T_I defines the time, which is necessary to accomplish the remainingrequired measurement time T_S−T_M, see also FIG. 11.

Whenever C0=T_I, then the forced measurements starts in order to ensurethat within T, measurements of total duration of at least T_S areaccomplished as required.

The forced measurements for IEEE802.11 systems can be accomplished bythe following method: The RDD announces a transmission to preferably adummy STA using the RTS/CTS method. The RTS-frame (and the CTS-frame)contains information about the required duration for the transmission.By this method all neighboring RLAN devices are informed about the timeand the duration of the next transmission, and all RLAN devices will besilent during this announced transmission.

Contrary to the usual and specified method, preferably no CTS need to besent back to the RDD (because the target was preferably ‘only’ a dummySTA). The RTS could also be sent to a real STA, which then responseswith a CTS frame. But for the current invention it is only necessary tosilence the STA, which are in the communication range to the RDD. I.e.no hidden station problem exists. The RDD announces sufficient time inthe RTS command, which is necessary for the rest of the radarmeasurements. If the required measurement time T_S−T_M is longer thanthe max. time allowed for a continuous transmission, then the RDD has topartition the rest of the measurement time and has to use several RTStransmissions as close together than possible.

During the time reserved by the RDD for a transmission, the RDD does nottransmit but will only measure. Therefore, this measurement is notdisturbed by other RLAN devices (with some rare exceptions that an RLANdevice has not received the RTS). Because the RTS transmission requesthas to use the standardized competition period within DCF, the access onthe transmission channel may be delayed if the traffic load in theneighborhood is high. Therefore, the time T_I can only roughly assessed.It is therefore proposed to give an RLAN device, which has to detectradar, a higher priority during the competition period than other RLANdevices. It is further proposed to control this priority by thedefinition of a new inter frame space RIFS (Radar Inter Frame Space).RIFS shall be shorter than DIFS, but larger than SIFS. Possibly it isequal to PIFS. I.e. SIFS<RIFS<=PIFS<DIFS. In this case no additionalframe space has to be specified, only that the RDD or another device,which silences the medium is allowed to us PIFS to get access to themedium, has to be specified.

The invention is not limited to the embodiments described above, but canbe varied freely within the scope of the appended claims. It is forexample entirely within the scope of the invention to apply theinventive principles to a system other than the IEEE 802.11, or to latergenerations of the IEEE 802.11 system.

1. A method for use in a wireless LAN-system of the 802.11-type foravoiding interference between radar signals and the signals exchangedbetween a plurality of wireless broadcasting nodes in the system, themethod comprising the steps of: enabling one of the nodes in the systemto function as a radar controlling node (RCN) and letting said RCN carryout measurement on at least one frequency in a frequency band which hasbeen assigned to the system, said measurements being carried out todetect if said at least one frequency is being utilized by a device or asystem foreign to said wireless communications system, such as a radardevice or system, the measurement being enabled to be carried out bymeans of the RCN transmitting a “quiet” message to other nodes in thesystem, said “quiet” message being a message which prohibits other nodesfrom transmitting during a certain interval defined in the “quiet”message, which method is characterized in that: said “quiet” message iscontained in the beacon message of the 802.11-system, as an InformationElement (IE) in the Beacon message, the “quiet” message which istransmitted contains information about the start of said interval, aswell as the duration of the interval, the “quiet” message is transmittedredundantly in each Beacon message by the RCN.
 2. The method of claim 1,according to which the redundancy is achieved by means of each “quiet”message within a Beacon message containing indications of multiple“quiet” intervals for different but consecutive Beacon intervals.
 3. Themethod of claim 2, according to which the redundancy is further enhancedby cycling fields with information about “quiet” intervals within theBeacon message over time.
 4. The method of claim 1, according to which“quiet” intervals are indicated by indicating a state for apseudo-random generator in each Beacon message, each station thensynchronizing respective pseudorandom generators to said state in orderto derive the start of the “quiet” intervals, the duration also beingindicated in the Beacon or agreed upon previously.
 5. The method ofclaim 4, according to which the start of the “quiet” interval isdetermined by the equation:T _(Quiet) _(—) _(Offset) =T _(Quiet) _(—) _(Duration)×Rem(State,floor(T _(Beacon) _(—) _(Interval) /T _(Quiet) _(—) _(Duration))) whereT_(Offset) is the offset time, State is the random generator state,T_(Duration) is the quiet time duration, T_(Beacon) _(—) _(Interval) isthe interval between the Beacons and Rem is the remainder function.
 6. Amethod for use in a wireless LAN-system of the 802.11-type for avoidinginterference between radar signals and the signals exchanged between aplurality of wireless broadcasting nodes in the system, the methodcomprising the steps of: enabling one of the nodes in the system tofunction as a radar controlling node (RCN) and letting said RCN carryout measurement on at least one frequency in a frequency band which hasbeen assigned to the system, said measurements being carried out todetect if said at least one frequency is being utilized by a device or asystem foreign to said wireless communications system, such as a radardevice or system, the measurement being enabled to be carried out bymeans of the RCN transmitting a “quiet” message to other nodes in thesystem, said “quiet” message being a message which prohibits other nodesfrom transmitting during a certain interval defined in the “quiet”message, which method is characterized in that the RCN recognizes radarsignals by means of measuring the duration of pulses received during the“quiet” intervals, so that if a received frame has a duration which isshorter than the shortest duration of a frame of the LAN-system, and hasa signal strength above a certain predetermined level, radar is assumed,and if the duration of the frame is longer than the longest assumedduration of a radar frame, LAN is assumed.
 7. The method of claim 6,according to which the signal strength of the received pulse iscalculated by means of a averaging using a first averaging process forreceived pulses which have a first predetermined duration, and a secondaveraging process for received pulses which have a second, shorter,predetermined duration.
 8. A method for use in a wireless LAN-system ofthe 802.11-type for avoiding interference between radar signals and thesignals exchanged between a plurality of wireless broadcasting nodes inthe system, the method comprising the steps of: enabling one of thenodes in the system to function as a radar controlling node (RCN) andletting said RCN carry out measurement on at least one frequency in afrequency band which has been assigned to the system, said measurementsbeing carried out to detect if said at least one frequency is beingutilized by a device or a system foreign to said wireless communicationssystem, such as a radar device or system, the measurement being enabledto be carried out by means of the RCN transmitting a “quiet” message toother nodes in the system, said “quiet” message being a message whichprohibits other nodes from transmitting during a certain intervaldefined in the “quiet” message, which method is characterized in that:the transmitting of the “quiet” message is triggered by the RCNdetecting that it needs a certain amount of time to carry out itsmeasurement, or by the RCN detecting an event which makes the radardetection too uncertain.
 9. The method of claim 8, in which said eventcomprises the number of detected corrupted LAN frames within a certaintime period exceeding a certain pre-defined level.
 10. The method ofclaim 8, in which said event comprises the detection within apre-defined period of time of an amount of radar pulses which is above apredefined threshold.
 11. A method for use in a wireless LAN-system ofthe 802.11-type for avoiding interference between radar signals and thesignals exchanged between a plurality of wireless broadcasting nodes inthe system, the method comprising the steps of: enabling one of thenodes in the system to function as a radar controlling node (RCN) andletting said RCN carry out measurement on at least one frequency in afrequency band which has been assigned to the system, said measurementsbeing carried out to detect if said at least one frequency is beingutilized by a device or a system foreign to saidwireless-communication-system, such as a radar device or system,characterized in that: the RCN tracks the whole LAN traffic withoutactively controlling the traffic, and, at the same time the RCN scansfor radar transmissions in time periods without traffic, with saidscanning being performed by RSS measurements, which are compared with apredefined radar detection power threshold, and: if said threshold isnot exceeded it is assumed that no radar is present. if said thresholdis exceeded, the RCN checks whether this was due to LAN traffic, and ifthis is the case, the corresponding LAN traffic duration is excludedfrom the measurement time.
 12. The method of claim 11, furthercharacterized in that if the LAN traffic load is during a desiredmeasurement period exceeds a certain level, or if the received LANtraffic exceeds a predefined radar power threshold more frequently thana certain limit, the RCN starts to control the LAN traffic, in order toensure that the radar measurements are no longer disturbed by LANtraffic.
 13. The method of claim 11, further characterized in that ifsignals received on a certain frequency cannot be detected as LANtraffic and are above a predefined radar power threshold, the RCN startsto initiate and control that this frequency is no longer used.
 14. Themethod of claim 11, in which the RCN starts to control the traffic tosuppress LAN traffic if it notices a certain amount of RSS measurementsabove a predefined radar power threshold although no LAN traffic isdetected, by means of which the RCN can ensure that it does not falselydetect radar due to non-detectable LAN interference.
 15. A method foruse in a wireless LAN-system of the 802.11-type for avoidinginterference between radar signals and the signals exchanged between aplurality of wireless broadcasting nodes in the system, the methodcomprising the steps of: enabling one of the nodes in the system tofunction as a radar controlling node (RCN) and letting said RCN carryout measurement on at least one frequency in a frequency band which hasbeen assigned to the system, said measurements being carried out todetect if said at least one frequency is being utilized by a device or asystem foreign to said wireless communications system, such as a radardevice or system, characterized in that: the RCN schedules measurementperiods on its current operating channel, as well as on other channels,including channels which have previously been detected as used by radardevices, and keeps track of the total measurement time (T_(TOT)), saidtotal measurement time being defined as the sum of all measurementintervals (T_(meas)) on a specific channel, and, the variable T_(TOT) isinitially set to a predetermined value once radar has been detected on aspecific channel, and each time a measurement is made on a channelpreviously detected as radar-occupied, if radar is detected during ameasurement interval, the value of T_(TOT) is unchanged, and if no radarsignal is detected during a measurement interval, the total measurementtime is increased in a cumulative manner, as T_(TOT)=T_(TOT)+T_(meas),and if T_(TOT) satisfies a given requirement on total measurement time,a previously radar-marked channel can be considered as radar-free, andif radar is detected after a number of radar-free measurements, thecumulative total time T_(TOT) will once again be set to its initialpredetermined value.